The purpose of this project is to characterize P-glycoprotein phosphorylation and the associated Ca++ and phospholipid-dependent protein kinase C (PKC) activities in multidrug resistant cells. The overexpression of PKC is closely associated with the mdr phenotype in both leukemic and breast carcinoma cell lines. Our experiments showed that PKC activity was highly elevated in adriamycin-resistant HL60 cells in comparison to the parental cell line and PKC-gamma isoform is present in HL60/AR cells but not in wild type cells. It was also noted that HL60/AR cells and not the parental cells phosphorylated vinculin in vitro. These data suggest that PKC-gamma or its catalytic fragment may have been responsible for vinculin phosphorylation. A second part of this project is to study the role of protein kinase C in MCF-7/adriamycin-resistant cells and BC19 cells which were transfected with the mdr gene. We have demonstrated that when BC19 cells are transfected with the protein kinase C alpha gene, resistance to adriamycin is increased several-fold; increased resistance is associated with enhanced phosphorylation of P-glycoprotein and decreased drug accumulation. These results suggest that phosphorylation of P-glycoprotein may serve as an important pharmacological target for reversing the multidrug-resistant process; protein kinase C may modulate the level of resistance to some anticancer drugs.